The invention relates to a combustion engine, in particular a gas engine, for a vehicle, in particular for a commercial vehicle, a method for operating a combustion engine, in particular a gas engine, and a vehicle, in particular a commercial vehicle, having the combustion engine and/or for carrying out the method.
To satisfy exhaust gas regulations that have to be met, one known practice is to convert gas engines or fuel gas engines, e.g., natural gas engines, from a lean-mix mode of operation to a stoichiometric mode of operation. A cooled and controlled exhaust gas recirculation system (EGR), which feeds back some of an exhaust gas from the gas engine to an intake of the gas engine, for example, is suitable for compensating the resulting loss of efficiency of the gas engine or for increasing the efficiency of the gas engine. During the subsequent combustion in a cylinder-piston unit of the gas engine, the recirculated exhaust gas acts substantially as an inert gas.
In gas engines without EGR and with the fuel gas being fed into an intake duct of the gas engine, a charge mixture mass flow fed to the cylinder-piston unit is generally calculated by a volumetric efficiency model implemented in a control unit, which reflects the displacement of the engine as a function of engine speed. For this purpose, the charge mixture pressure (manifold absolute pressure or MAP) and the charge mixture temperature in the intake duct are first of all measured in the region of the cylinder-piston unit by a pressure sensor and a temperature sensor. Using the volumetric efficiency model, the charge mixture mass flow supplied can be calculated from the measured charge mixture pressure, the charge mixture temperature, and the speed of the engine. Using the combustion air ratio, an air mass flow fed to the cylinder-piston unit can then also be calculated. However, this procedure is not possible in the case of gas engines with EGR since the exhaust gas fraction in the charge mixture formed, for example, as a fuel gas/air/exhaust gas mixture is not known.
In the case of gas engines with EGR, the usual practice for determining the air mass flow is therefore to measure the air mass flow in the intake duct by an air mass meter, e.g., by a hot film air mass meter, upstream of the EGR and upstream of the fuel gas supply. Using the measured air mass flow and of the charge mixture mass flow determined by the volumetric efficiency model already mentioned, the recirculated exhaust gas mass flow can then also be determined. However, measurement of the air mass flow by the air mass meter has proven very prone to faults, particularly owing to the sensitivity thereof to dirt.